1. Field of the Invention
The present invention relates to a heating apparatus of induction heating type for heating an image formed on a recording material.
2. Related Background Art
In an image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus, there is widely employed a heating apparatus of contact heating type such as heat roller type, film heating type or magnetic induction heating type, as an image heating apparatus represented by a heat fixing apparatus for fixing, to a recording material to be heated such as a transfer sheet, a printing sheet, a photosensitive (electrofax) sheet or an electrostatic recording sheet, an unfixed visible image (unfixed toner image), formed by an appropriate image forming process such as an electrophotographic process, an electrostatic recording process or a magnetic recording process on such recording material to be heated and borne thereon by a transfer (indirect) method or a direct method.
The heating apparatus of heat roller type is basically composed of a pair of rollers consisting of a heat roller (fixing roller) heated and regulated to a predetermined temperature by a heat source such as a halogen lamp and a pressure roller maintained in pressure contact therewith, and the material to be heated is introduced into and conveyed in the contact nip (fixing nip) of the rollers whereby the unfixed visible image on the material to be heated is fixed thereto by the heat of the heat roller.
Also the heating apparatus of the film heating type is disclosed for example in the Japanese Patent Applications Laid-open Nos. 63-313182, 1-263679, 2-157878, 4-44075, 4-44075 to 4-44083; and 4-20498 to 4-204984 and is commercially used.
In such heating apparatus, the material to be heated is maintained in contact with a heating body fixedly supported by a support member across a thin heat-resistant film material (fixing film) and the film material is made to slide over the heating body thereby giving the heat of the heating body to the material to be heated across the film material. The heating body can be composed of so-called ceramic heater, basically composed of a ceramic substrate (hereinafter simply called substrate) of sufficient heat resistance, electrical insulation and heat conductivity such as alumina (Al2O3) or aluminum nitride (AlN) and a resistance layer formed on the surface of the substrate and capable of generating heat by electric current supply. Also the film material can be composed of a thin film of a low heat capacity. Therefore, in comparison with the heating apparatus of heat roller type, the heating apparatus of film heating type has a higher heat transmitting efficiency and a faster start-up, thereby enabling reduction of the waiting time (quick starting property or on-demand operability) and electric power saving.
Also in the image heating apparatus of magnetic induction heating type, as disclosed in the Japanese Patent Application Laid-open No. 7-319312, it is proposed to generate an eddy current in a metal core of the fixing roller by an alternating magnetic field, thereby generating Joule""s heat by magnetic induction in such metal core namely in the fixing roller.
As shown in FIG. 8, there are shown a cylindrical heating roller 1 provided rotatably in a direction a by an unrepresented driving source. and a pressure roller 2 maintained in pressure contact with the heating roller 1 and rotated by the rotation thereof. Inside the heating roller 1, there is provided a coil assembly 3 for generating an induction magnetic field. The pressure roller 2 is provided with a silicone rubber layer 6 around a shaft core 4. There are shown a thermistor 6 serving as a temperature sensor for detecting the surface temperature of the heating roller 1, and a separating claw 7 for separating sheet from the heating roller 1.
In the coil assembly 3, a square-shaped bobbin 8 with an aperture in the center is wound with a copper wire by plural turns in a single direction to constitute an excitation coil 9, and a core 10 is inserted into the central aperture 8a of the bobbin 8 in a direction perpendicular to the copper wire of the excitation coil 9. The coil assembly 3 is so positioned that the copper wire wound on the bobbin 8 extends along a plane parallel to the rotary axis of the heating roller 1, namely that the core 10 is perpendicular to the rotary axis, thereby generating magnetic flux in a direction perpendicular to the rotary axis of the heating roller 1.
Plural coil assemblies 3 are arranged along the axial direction of the heating roller 1 by means of a holder 11 in such a manner that the core is parallel to the conveying direction of the sheet and an end of the excitation coil 9 is opposed to the pressure roller 2 while the other end of the coil 9 is opposed to the thermistor 6. The holder 11 is composed of a heat-resistant plastic material, and has a diameter somewhat smaller than the internal diameter of the heating roller 1 in order to form a gap to the internal wall of the heating roller 1.
FIG. 9 is a block diagram of a circuit for supplying the induction heating coil 9 with a high frequency current thereby controlling the temperature of the heating roller 1. The AC current of a commercial power source 13 is rectified by a rectifying circuit 14 and is converted by an inverter circuit 20 into the high frequency current. The current to the inverter circuit 20 is supplied through a thermostat 15 constituting a temperature fuse maintained in contact with the surface of the heating roller 1.
The thermostat 15 serves to cut off the current supplied to the circuit when the surface temperature of the heating roller 1 reaches a predetermined abnormal temperature, and is provided, like the thermistor 6, in a position opposed to the excitation coil 9 wound on the core 10. A control circuit 16 is composed of a microprocessor, a memory etc., and provides a drive circuit 21 in the inverter circuit 20 with an on/off signal while monitoring the temperature of the heating roller 1 based on a potential corresponding to the temperature detected by the thermistor 6, thereby executing temperature control.
In the inverter circuit 20, when the control signal from the control circuit 16 is turned on, the drive circuit 21 turns on a switching element 22 consisting for example of a transistor, an FET or an IGBT thereby providing the excitation coil 9 with a current.
On the other hand, a current detecting circuit 22, upon detecting that the current reaches a predetermined current value IP, sends a signal to the drive circuit 21 so as to turn off the switching element 22. When the switching element 22 is turned off, a resonance current is generated between the excitation coil 9 and a resonance capacitor 24. Then, upon detecting that the voltage V of the switching element 22 at the side of the excitation coil 9 is lowered to about 0 V by the resonance, a voltage detecting circuit 25 sends a signal to the drive circuit 21 so as to turn on again the switching element 22.
In the heating apparatus of magnetic induction heating type of the above-described configuration, the temperature control at the heating position (nip portion) for the material to be heated is executed by employing a thermistor as the temperature detecting element, detecting the temperature by positioning such thermistor in contact with the surface or internal surface of the roller body or the film member constituting the heating body for the material to be heated (contact type temperature detection) and controlling the electric power supply from the driving power source to the excitation coil 9 by the control circuit 16 based on the information of such temperature detection, thereby maintaining the temperature of the heating portion of the material to be heated at a predetermined value.
In such configuration, however, the exact temperature detection is hindered since the temperature detection signal outputted from the thermistor functioning as the temperature detecting element is superposed with induction noise components such as a noise resulting from the alternating magnetic field generated by the excitation coil serving as the magnetic field generating means and a switching noise resulting from the inverter circuit. As a result, there are generated an unevenness and a ripple in the temperature on the surface of the fixing roller, leading to the deterioration of the fixing ability.
In consideration of the foregoing, an object of the present invention is to provide an induction heating apparatus capable of temperature control of satisfactory accuracy.
Another object of the present invention is to provide an induction heating apparatus capable of providing a satisfactory precision in the signal entered from a temperature detecting element to temperature control means.
Still another object of the present invention is to provide an induction heating apparatus capable of eliminating noises from the signal entered from a temperature detecting element to temperature control means.
Still another object of the present invention is to provide an induction heating apparatus comprising:
a heating member;
an excitation coil for generating a magnetic field to induce an eddy current in the heating member;
a temperature detecting element for detecting the temperature of the heating member; and
control means for controlling an electrical supply to the excitation coil in accordance with the temperature detected by the temperature detecting element;
wherein the output of the temperature detecting element is inputted into the control means through a band-pass filter.
Still other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings.